Currently, run-flat tires that have a high-strength sidewall reinforcing layer (insert) arranged on the inner side of the sidewall portion have been developed for practical use, so that the vehicle can travel a certain distance even when air pressure is lost due to a puncture (zero internal pressure). Therefore, it is no longer necessary to always carry a spare tire, which should allow a reduction in the weight of the overall vehicle. However, the running speed and the distance that the vehicle can travel on a puncture are limited, so that there is a need for further improvement in the durability of run-flat tires.
An example of a method for improving durability of a run-flat tire is to prevent the tire from breaking apart by making the reinforcing layer thicker in order to suppress deformation. However, since this increases the tire weight, such a method goes against the original objective of a run-flat tire, which is to reduce weight.
Another method is to suppress deformation by increasing the hardness of the reinforcing layer by increasing the amount of a reinforcing filler such as carbon black. However, this increases the load on a kneading machine during steps such as kneading and extrusion, and also increases heat build-up in terms of the post-vulcanization physical properties. Consequently, this method cannot be expected to provide much of an increase in run-flat durability, and also suffers from the problem that fuel economy deteriorates. Further, to improve run-flat durability, it is also important to improve the breaking properties (e.g. elongation at break) at high temperature.
Patent Document 1 considers adding sulfur and two or more vulcanization accelerators at predetermined amounts. However, the rubber composition shows a small elongation at high temperature, and therefore reduced tensile strength. Consequently, there is still a need for further improvement in reducing heat build-up, and enhancing elongation at break at high temperature, and increasing run-flat durability, while good rigidity is maintained. In addition, there is also a need to resolve problems such as reduced processability and storage stability.    Patent Document 1: JP 2002-155169 A